Process for the preparation of a dental prosthesis by slight solid phase fritting of a metal oxide based infrastructure

ABSTRACT

A method for manufacturing a dental prosthesis from a shaped, slightly solid phase fritted metal oxide infrastructure whose pores are impregnated with a glass by molding a tooth model from a linearly expanding molding mass and contacting the model with a slip of metal oxide particles which particles are agglomerated onto the model. The agglomerated metal oxide particles forming an infrastructure which is shaped and baked to effect dehydration and withdrawal of the model from the infrastructure before the fritting and glass impregnation of the infrastructure.

The present invention relates to a new method for manufacturing dentalprostheses, as well as to prostheses obtained by this method.

Dental prostheses are generally defined as any part designed to befitted onto the denture of a patient, for the purpose of restoring thedenture to its natural shape, in whole or in part.

Hence, prostheses manufactured according to the present invention cancomprise, for instance, peripheral caps or crowns that are placed on thestump of a natural tooth, or even prostheses that are generallydesignated as "inlay" or "onlay" which are indented to reconstitute apartial alteration of a tooth by filling a cavity, resulting from a lossof tooth substance, with a piece of the same form produced by theprosthesis, or even bridges, which are prostheses, that are supportedsimultaneously on the remaining parts of at least two teeth bycompensating ultimately for one or several missing teeth.

Methods are known for manufacturing such prostheses which are most oftenmetallic parts that are fusion molded, on which can be fixed, at a hightemperature, an enamel which gives the prosthesis the appearance of anatural tooth.

These known prostheses exhibit the disadvantage of being delicate andcostly to manufacture. Furthermore, because of their metallic contentthey are not entirely biocompatible, and a corrosion phenomenon of theprostheses has been observed which requires the replacement of thelatter.

The present invention envisages the production of dental prostheseswhich are completely ceramic-based and which exhibit excellentmechanical characteristics which are comparable to those of naturalteeth.

The prostheses which are manufactured in accordance with the presentinvention are biologically compatible with ceramics. They cost less tomanufacture than known prostheses, and they can be fitted with greataccuracy onto those parts of natural teeth where they are to be affixed.

It is known that "ceramics" are defined in the dental art not only asthe products produced from terra cotta but also those which includeenamels and metallic oxides such as aluminum oxide or zirconium oxide."Ceramometallic prostheses" are those which are produced in part usingmetals that are generally found in the form of alloys.

The purpose of the present invention is to provide a method formanufacturing ceramic dental prostheses, characterized by the fact thata model of a tooth that is to receive the prosthesis is produced in amolding mass, such as plaster, which exhibits slight linear expansionduring solidification; that a slip is prepared comprising a suspensionin water of metal oxide particles such as aluminum oxide and/orzirconia, to which is added a suspension stabilization agent andoptionally a pH control agent; that the model of the tooth previouslyproduced is contacted with the slip so that the metal oxide particlesaggregate on the surface of the model of the tooth, until a sufficientthickness is obtained; that there is imparted to the layer of metaloxides, the outer shape that is desired for the infrastructure of theprosthesis; that the model of the tooth coated with the metaloxide-based infrastructure is baked in order to effect initiallydehydration of the model of the tooth which causes its withdrawal, atwhich point the solid phase of the metal oxide particles is slightlyfritted; and that the infrastructure thus fritted is impregnated withglass at a temperature sufficient so that the glass occupies all of theopen pores which exist in the infrastructure.

According to a preferred embodiment of the present invention, the outersurface of the prosthesis is coated with an enamel that is compatiblewith the nature of the glass used for impregnating the infrastructureand which has a coefficient of expansion close to that of the latter.

Thus, the method of the present invention comprises a first step ofproducing an infrastructure or skeleton which comprises an aggregationof metal oxide particles brought together by slight fritting in a solidphase and then in a second step, filling all of the existing intersticesbetween the metal oxide particles with melted glass which completelyfills the uninterrupted network of cavities left by the fritted metaloxide particles.

In other words, the prostheses according to the present invention aremade by the total interlacing of two uninterrupted networks of which onecomprises fine fritted metal oxide particles in the solid phase and theother comprises glass.

The dental prostheses obtained according to the present inventionexhibit excellent mechanical features. Surprisingly, their mechanicalfeatures are indeed superior to those that can be obtained by completefritting of metal oxide particles without impregnating them with glass.

The molding mass intended, in accordance with the present invention, foruse in producing the model of the tooth, can be an alpha-type plastercomprising calcium sulfate hemihydrate, CaSO₄.1/2H₂ O without anymineral-based charge. This plaster must exhibit a linear solidificationexpansion (or setting), preferably, between 0.1% and 0.4%. Thisexpansion increases with the quantity of plaster added to a givenspecific quantity of water in order to produce the molding. It alsoincreases with the addition of sodium chloride (NaCl) or celluloseether.

This molding mass can also comprise refractory charges such as siliconoxide or aluminum oxide, admixed with a binding agent, such as sodiumsilicate, ethyl silicate, ammonium sulfate, aluminum phosphate or sodiumphosphate, or ammonium acid phosphate.

In accordance with the present invention, the molding mass for the modelof the tooth must exhibit during its solidification a slightly linearexpansion which can range, for instance, from 0.1% to 0.4%.

In a preferred embodiment of the present invention, the metal oxideemployed comprises an alumina powder (Al₂ O₃) which includes,optionally, in admixture a zirconium oxide (ZrO₂) powder in amounts thatcan be significant.

There can also be employed, in accordance with the present invention,zirconium oxide that is pure or that is stabilized with yttrium oxide.

The slip can be obtained according to the present invention, by placing,for instance, 100 g of metal oxide particles in about 12 to 20 g ofwater and by adding between 0.05 g and 0.5 g of a suspension stabilizingagent, which agent can comprise, for instance, a polyvinyl alcohol, anacrylic acid, a cellulose ester, or sodium silicate.

In accordance with the present invention it is also preferable to reducethe pH of the slip to an essentially neutral value, for instance, to avalue of about 7 to 8, by the addition of an appropriate substance suchas citric acid.

Before using the slip according to the invention, it is preferable todegas the latter, by subjecting it to ultrasonics in a receptacle wherea vacuum is created.

In accordance with the present invention, the first baking of theinfrastructure, made up of the layer of metal oxide particles appliedonto the tooth model, has as its first goal the separation of those twoelements by withdrawal of the molding mass that comprises the model ofthe tooth, which in the case of plaster is obtained by the loss of atleast part of the constituent water of gypsum found in plaster.

Such dehydration of the plaster can be obtained by putting theinfrastructure of metal oxide particles placed on the plaster model ofthe tooth inside an oven where the temperature is raised slowly, forinstance about 1° C. per minute, until it reaches a temperature of about180° C., then later in a more rapid manner up to about 330° C.

In this first heat treatment phase, the plaster separates from thepowder metal oxide infrastructure.

In accordance with the present invention, a slight fritting of the metaloxide particles in a solid phase must be carried out so as to obtain arigid skeleton, with open porosity, which is subjected during frittingto a slight withdrawal which is less, for example, than 0.4%, and whichis compensated by the expansion that the model of the tooth undergoesduring the solidification of the mass.

The present invention thus provides prostheses that adapt to the naturaltooth with a great deal of accuracy.

In accordance with the present invention, it is advantageous for thisfritting to be only a slight fritting, which simply produces a bindingbetween the metal oxide particles without causing a substantialreduction of the infrastructure volume.

In accordance with the present invention, the heat treatment that makesit possible to attain the desired result, depends on its temperature, onthe speed of temperature elevation, on the period of time during whichthe temperature is maintained and on the average size of the metal oxideparticles.

Thus, for instance, for a heat treatment of about one to three hours,the temperature can be about 1050° C. to 1150° C. with metal oxideparticles having an average size of about 3.5 microns.

The treatment temperature can be raised to 1250° C. with metal oxideparticles having an average size of about 8.5 microns, and from 1300° C.to 1400° C. with metal oxide particles having an average size of about20 microns.

The effect of an increase in the rate of temperature rise is thereduction of the withdrawal that occurs during fritting.

In accordance with the present invention, it is preferable not to usemetal oxide particles which have too high a specific surface.Advantageously, there can be employed powders of metal oxide particles,having for instance, a specific surface of about 1 to 5 m² per g.

It is possible to raise the fritting temperature, while maintaining aslight withdrawal, by admixing with the aluminum oxide and/or zirconiumoxide powder, other metal oxide powders such as magnesium oxide (MgO)powder. Thus, an addition of about 0.3 to 3% of magnesium oxide powdercan make it possible to increase the fritting temperature by about 150°C.

There can also be used for this same purpose an addition of lanthanumoxide powder (La₂ O₃) or even powders of other oxides such as yttriumoxide (Y₂ O₃)or rare earth oxides.

Fritting of the metal oxide powders at a fairly low temperature, forinstance at a temperature between 1050° and 1150° C., provides theadvantage of being able to work with ovens that are analogous to thosecommonly used in the dental art.

An elevation of the fritting temperature of the metal oxide particlesprovides the advantage of being able to impregnate the infrastructurewith glass by operating at a higher temperature, which permits a greaterdiversity in the selection of the glass as will be explained in greaterdetail below. The glass impregnation of the fritted metal oxide skeletonis effected without having to increase the withdrawal resulting fromfritting which is achieved during the first heat treatment. This meansthat impregnation with the glass must be carried out at a temperaturewhich is at best equal to the fritting temperature during a relativelyshort period of time which in practice is preferably not in excess of 2to 4 hours, but which depends on the thickness to be impregnated.

The glass used to effect the impregnation of the skeleton, constitutedby the fritted metal oxide powder, must exhibit a number ofcharacteristics.

Although the pores of the fritted metallic oxide skeleton, according tothe present invention, can have a size as small as 0.3 micron, it ispreferable, in accordance with the invention, that the glass impregnatesall of the pores of the skeleton.

To this end, it is preferable that the glass exhibit at the treatmenttemperature, the characteristics of wetting the fritted metallic oxideskeleton, which means that the surface energy of the glass, at thistemperature, must be lower than the surface energy of metal oxideparticles.

The wetting character of the glass can be increased in accordance withthe present invention by introducing in the composition, for instance,boron oxide, lead oxide or vanadium oxide.

The glass must exhibit preferably, at the considered temperature, a lowviscosity, which can be obtained by an appropriate selection ofproportions of various oxides, by increasing, for example, the contentof boron oxide, lead oxide or lanthanum oxide.

The reactivity of the glass with regard to the metal oxide must beneither too strong, nor too weak. This is obtained by using a glasswhich contains initially metal oxides such as Al₂ O₃ and/or ZrO₂ in anamount that is slightly lower than, but close to, the saturation of theglass vis-a-vis those metal oxides at the impregnation temperature.

The coefficient of expansion of the glass must, preferably, be lowerthan, but close to, the coefficient of expansion of the skeleton offritted metal oxide particles, so as to obtain good resistance againstheat shocks for the prosthesis.

The coefficient of expansion of the glass can be adjusted for instance,by adding sodium oxide, potassium oxide or lithium oxide in order toincrease it, or by adding silicon oxide or titanium oxide to reduce it.

The impregnation glass can contain a small amount of metal oxides ormetals, for instance up to 2%, in order to provide color to theprosthesis and thus alter the color of the fritted metal oxide skeleton.

By selecting an impregnation glass whose index of refraction is more orless close to that of the fritted infrastructure, a more or lesstransluscent prosthesis is obtained. In this fashion, the index of therefraction of the impregnation glass can be varied in order to obtaindifferent optical effects.

Furthermore, it is important to note that the impregnation glasscomprises mainly, a mixture of oxides which, after solidification areprovided in a completely amorphous state by being transparent or in amore or less crystalline state by being opalescent.

In order to carry out the impregnation of the fritted metal oxideskeleton with glass, several techniques can be used in accordance withthe present invention.

For instance, there can be applied onto the outer surface of theinfrastructure of the prosthesis (in other words, on the surface of theprosthesis, whose shape has not been determined by the model of thetooth), a water-based glass paste which, when heated to the appropriatetemperature, melts and spreads spontaneously inside the entire volume ofthe infrastructure by filling all of the pores.

Further, in accordance with the present invention, there can be placedin a cupel, a bed of glass powder, on which the infrastructure is placedand then the whole is heated to the desired temperature. When the glasshas melted, it occupies, through capillarity, the entire network ofpores in the fritted metal oxide particle infrastructure.

It is preferable that the surface of the prosthesis which comes intocontact with the tooth should not contact the melted glass and that thefilling of the pores located in the vicinity of this surface is effectedby capillarity starting from the interior of the infrastructure mass offritted metal oxide particles, so that no excess glass remains on thesurface, which otherwise would alter its geometry.

It is advantageous to cover the infrastructure comprising the frittedmetal oxide particles impregnated with glass by one or more layers ofenamel having optical properties and different tints, so as to give theprosthesis the appearance of natural teeth.

In this case, the mass of metal oxide particles that covers the model ofthe tooth is modeled before fritting, in order to impart to it a shapethat provides room necessary for the enamelling that intervenes at theend of the process according to the invention.

It is appropriate to use preferably, as an enamel, a charged glass whichhas a coefficient of expansion that is slightly lower than that of theinfrastructure and whose shaping is effected starting with a powdermixed with water that is sculpted and whose mechanical consolidation isobtained by fritting in liquid phase.

Advantageously, an enamel of the alkali borosilicate type, whichcontains alumina, is employed.

The present invention also relates to a dental prosthesis obtained bythe above described method and which is characterized by the fact thatit comprises an infrastructure (or skeleton) obtained by fritting, insolid phase, metal oxide particles and comprising an uninterruptednetwork, whose interstices are filled with glass.

The following nonlimiting examples are given to illustrate theinvention.

EXAMPLE 1 Production of a peripheral cap or crown

Initially, in accordance with conventional techniques the tooth ismachined and impressions are taken which make it possible to obtain aninitial working model which restores, in positive, the shape of thetooth which is to receive the crown, as well as the shape of theadjacent and opposing teeth.

A second model of the tooth which must receive the crown is produced ina plaster comprising a mixture of 100 g of calcium sulfate hemihydrateand 21 cc of water.

When the plaster, representing the model of the tooth, has set and hasdried, it has undergone an expansion of about 0.4%. It is then immersedin a slip, in accordance with the present invention, which comprises 100g of alumina powder, wherein the average grain size is 3.5 microns,admixed with 13 cc of water containing 0.5 g of cellulose ester, the pHbeing adjusted to 7.6 by the addition of 0.07 g of citric acid.

Water absorption by the capillaries of the plaster causes anagglomeration of alumina particles onto the surface of the model,according to a thickness which is a function of the period of immersionin the slip, and which can be, for example, about 0.5 to 1 mm or more ifnecessary.

The thickness of the alumina deposit can be altered either with a brushby bringing some slip to the parts whose thickness is to be increased,or with the aid of a spatula whereby excess deposit can be removed andthe edges can be finished.

Therefore, it is possible to proceed easily with sculpting theprosthesis as soon as the alumina deposit has developed essentially theconsistency of clay.

Then the plaster model which supports the alumina layer thus produced,is placed in an oven, preferably under vacuum, to facilitate drying,unless it is preferred to let it dry in the open air.

The whole is then placed in a stove where the temperature is initiallyraised to about 180° C. at a rate of about 1° C. per minute. Thetemperature is then raised to about 330° C., the entire length of theoperation being staged over three to five hours, for example.

The elimination of the constituent water from the plaster and aseparation of the model from the alumina infrastructure are thusobtained.

Without removing the ensemble from the stove, the temperature isgradually increased, for example, in about 1 hour, to about 1100° C., atemperature which is maintained for about two hours.

After having allowed cooling to take place, it is noted that the plastermodel withdraws substantially, and that the fritted alumina particleinfrastructure is sufficiently consolidated so that it can be handled.

The resulting fritting is effected with a withdrawal of 0.3% which iscompensated by the expansion of the plaster of the model of the toothduring its setting, taking into account a slack of 0.1% for theplacement of the prosthesis.

In order to perform the impregnation in accordance with the presentinvention using melted glass, the fritted alumina infrastructure isplaced inside a cupel on a bed of glass powder whose composition, byweight, is as follows:

    ______________________________________                                        Silica (SiO.sub.2)  20 g                                                      Boron oxide (B.sub.2 O.sub.3)                                                                     19 g                                                      Aluminum oxide (Al.sub.2 O.sub.3)                                                                 20 g                                                      Lanthanum oxide (La.sub.2 O.sub.3)                                                                30 g                                                      Calcium oxide (CaO)  5 g                                                      Titanium oxide (TiO.sub.2)                                                                         4 g                                                      Coloring oxides      2 g                                                      ______________________________________                                    

The glass is gradually heated to a temperature slightly lower than 1100°C. which is maintained for two to three hours, so as to enable themelted glass to penetrate, by capillarity, the alumina particleinfrastructure, thereby filling all the pores.

According to the present invention, the crown thus produced is enameledby applying several layers of enamel whose composition, by weight, is asfollows:

    ______________________________________                                        Sodium oxide (Na.sub.2 O)                                                                        4.6 g                                                      Potassium oxide (K.sub.2 O)                                                                      7.6 g                                                      Calcium oxide (CaO)                                                                              1.7 g                                                      Aluminum oxide (Al.sub.2 O.sub.3)                                                                13.9 g                                                     Silica (SiO.sub.2) 65.5 g                                                     Boron oxide (B.sub.2 O.sub.3)                                                                    6.7 g                                                      ______________________________________                                    

Coloring additives and charges such as tin oxide, silica (quartz),zirconium oxide or aluminum oxide can vary with successive layers.

When the prosthesis in accordance with the present invention is notintended to be enameled, it is clear that the deposit of aluminaparticles on the plaster model of the tooth, must be given a shape whichcorresponds to the outer shape that is desired to be imparted to theprosthesis. This is obtained by modeling or sculpting the deposit ofalumina particles according to conventional techniques for dentalprostheses.

EXAMPLE 2 Production of a peripheral cap or crown

The procedures of Example 1 are repeated, except that the second modelof the tooth which must receive the crown is produced using a mixturehaving the following composition:

    ______________________________________                                        Refractory charge of Silica (SiO.sub.2) or                                                             75 g                                                 of alumina (Al.sub.2 O.sub.3)                                                 Magnesium oxide (MgO)    10 g                                                 Ammonium acid phosphate (NH.sub.4 H.sub.2 PO.sub.4)                                                    15 g                                                 Water                    24 g                                                 ______________________________________                                    

The slip comprises a mixture having the following composition:

    ______________________________________                                        Aluminum oxide (Al.sub.2 O.sub.3)                                                                     97 g                                                  Magnesium oxide (MgO)   3 g                                                   Water                   17 g                                                  Polyvinyl alcohol       0.25 g                                                ______________________________________                                    

The metal oxides have an average grain size of 8 microns. The pH of thecomposition is adjusted to 8 by the addition of picric acid.

Fritting is carried out by heating the model of the tooth coated withthe layer of metal oxides to a temperature of 1250° C. for 1 hour.

Impregnation of the resulting infrastructure is carried out bycontacting it for a period of two hours at a temperature of 1200° C.with a glass having the following composition:

    ______________________________________                                        Silica (SiO.sub.2)       10 g                                                 Boron oxide (B.sub.2 O.sub.3)                                                                          12.5 g                                               Alumina (Al.sub.2 O.sub.3)                                                                             20 g                                                 Yttrium oxide (Y.sub.2 O.sub.3)                                                                        20 g                                                 Lanthanum oxide (La.sub.2 O.sub.3)                                                                     25 g                                                 Titanium oxide (TiO.sub.2)                                                                             5 g                                                  Calcium oxide (CaO)      5 g                                                  Cerium oxide (CeO)       2.5 g                                                ______________________________________                                    

EXAMPLE 3 Production of a peripheral cap or crown

The procedures of Example 1 are repeated and the second model of thetooth is effected using the following composition:

    ______________________________________                                        Refractory charge of silica (SiO.sub.2) or                                                             75 g                                                 alumina (Al.sub.2 O.sub.3)                                                    Magnesium oxide          10 g                                                 Ammonium acid phosphate (NH.sub.4 H.sub.2 PO.sub.4)                                                    15 g                                                 Water                    24 g                                                 ______________________________________                                    

This model is immersed in a slip having the following composition:

    ______________________________________                                               Alumina (AlO.sub.3)                                                                            300 g                                                        Water            20 g                                                         Sodium silicate  0.5 g                                                 ______________________________________                                    

The alumina particles have an average grain size of 20 microns.

Fritting is carried out at a temperature of 1300° C. for about 2 hours.

Impregnation is achieved with a glass having the following composition:

    ______________________________________                                        Silica (SiO.sub.2) 63 g                                                       Alumina (Al.sub.2 O.sub.3)                                                                       14 g                                                       Calcium oxide (CaO)                                                                              23 g                                                       ______________________________________                                    

EXAMPLE 4 Production of a peripheral cap or crown

The procedures of Example 1 are repeated and the plaster model of thetooth is immersed in a slip having the following composition:

    ______________________________________                                        Alumina (Al.sub.2 O.sub.3)                                                                             80 g                                                 Zirconium oxide (ZrO.sub.2)                                                                            20 g                                                 stabilized with Yttrium                                                       Water                    15 g                                                 Polyvinyl alcohol        0.1 g                                                ______________________________________                                    

The alumina particles have an average grain size of about 3.5 microns.

The zirconium oxide particles have an average grain size of 0.5 micron.

The pH is adjusted to 7 by the addition of citric acid.

Fritting is carried out for three hours at a temperature of 1150° C.

Impregnation of the fritted infrastructure is carried out with a glasshaving the following composition:

    ______________________________________                                        Silica (SiO.sub.2)       22 g                                                 Boron oxide (B.sub.2 O.sub.3)                                                                          15 g                                                 Alumina (Al.sub.2 O.sub.3)                                                                             20 g                                                 Zirconium oxide (ZrO.sub.2)                                                                            2 g                                                  Lanthanum oxide (La.sub.2 O.sub.3)                                                                     28 g                                                 Calcium oxide (CaO)      7 g                                                  Titanium oxide (TiO.sub.2)                                                                             3 g                                                  Iron oxide (Fe.sub.2 O.sub.3)                                                                          2 g                                                  Cerium oxide (CeO)       2.5 g                                                ______________________________________                                    

EXAMPLE 5 Production of a prosthesis involving several teethsimultaneously.

In order to produce a prosthesis that involves several teethsimultaneously, such as a "bridge", the procedures of Examples 1 through4 are repeated except that a model which includes different teeth thatmust receive the prosthesis is employed and the deposit of metal oxideparticles is sculpted as a function of the shape that is desired for theprosthesis.

In Examples 2 through 5, according to the present invention, enamelingcan be achieved in successive layers with glasses having the followingcomposition, by weight:

    ______________________________________                                        Sodium oxide (Na.sub.2 O)                                                                       4.7 to 4.2 g                                                Potassium oxide (K.sub.2 O)                                                                     8.2 to 6.8 g                                                Calcium oxide (CaO)                                                                             1.8 to 1.5 g                                                Aluminum oxide (Al.sub.2 O.sub.3)                                                               15 to 13 g                                                  Silica (SiO.sub.2)                                                                              62.8 to 68 g                                                Boron oxide (B.sub.2 O.sub.3)                                                                   7.5 to 6.5 g                                                ______________________________________                                    

EXAMPLE 6 Production of a partial reconstitution piece

These pieces which are usually known as "inlay" and "onlay" are used tofill a cavity which is carried out by stripping the tooth.

To this end, an impression is taken which makes it possible toreconstitute a positive model of the tooth made of plaster or anotherporous material. Using a brush, for instance, the slip is deposited inthe cavity of the model in such a way that the latter is filled with asufficient quantity of metal oxide particles that is sculpted to restorethe missing piece of the tooth.

The procedures outlined in the previous examples can be repeatedalthough in this situation it is generally not necessary to cover theresulting prosthesis with a layer of enamel.

What is claimed is:
 1. A method for manufacturing ceramic dentalprosthesis comprisingmolding a model of a tooth that is to receive saidprosthesis from a molding mass that slightly expands linearly onsolidification, preparing a slip comprising an aqueous suspension ofmetal oxide particles to which has been added a suspension stabilizationagent; contacting said model of said tooth with said slip until a layerof agglomerated metal oxide particles has been achieved, imparting tosaid layer of agglomerated metal oxide particles an outer shape desiredfor an infrastructure of said prosthesis, baking said model of saidtooth coated with said metal oxide based infrastructure to effectinitially a dehydration of said model of said tooth thereby causingwithdrawal of said model from said infrastructure, slightly solid phasefritting the metal oxide based infrastructure to produce a bindingbetween said metal oxide particles without causing a substantialvolumetric reduction of said infrastructure, and then impregnating saidinfrastructure with glass at a temperature such that all pores of saidinfrastructure are filled with said glass, thereby producing saidprosthesis.
 2. The method of claim 1 wherein said model of said toothexpands linearly between about 0.1 to 0.4 percent on soldification. 3.The method of claim 1 wherein said molding mass is a mineral charge freecalcium sulfate hemihydrate.
 4. The method of claim 1 wherein said slipcontains about 0.05 to about 0.5 weight percent of said metal oxideparticles and said suspension stabilization agent is polyvinyl alcohol,an acrylic acid, a cellulose ester or sodium silicate.
 5. The method ofclaim 1 which includes degassing said slip, prior to placing said modelof said tooth in contact therewith, by submitting said slip toultrasonics in a vacuum.
 6. The method of claim 1 wherein said frittingof said infrastructure is carried out at a temperature of about 1050° C.to 1150° C. when said metal oxide particles are alumina having anaverage size of about 3.5 microns.
 7. The method of claim 1 whereinfritting of said infrastructure is carried out at a temperature of about1250° C. when said metal oxide particles are alumina having an averagesize of about 8.5 microns.
 8. The method of claim 1 wherein saidfritting of said infrastructure is carried out at a temeperature ofabout 1300° C. to 1400° C. when said metal oxide particles are aluminahaving an average size of about 20 microns.
 9. The method of claim 1wherein said metal oxide particles have a specific surface of about 1 to5 m² / gram.
 10. The method of claim 1 wherein said glass employed toimpregnate said infrastructure has a surface energy at said impregnationtemperature lower than a surface energy of said infrastructure aftersaid fitting.
 11. The method of claim 1 wherein said glass contains leadoxide, boron oxide or vanadium oxide in an amount sufficient to increasethe wettability of said glass.
 12. The method of claim 1 wherein saidglass comprises same metal oxides employed in said infrastructurewherein said metal oxides are present in amount slightly less than asaturation of said glass relative to said metal oxides at saidimpregnation temperature.
 13. The method of claim 1 wherein said glasshas a coefficient of expansion lower than a coefficient of expansion ofsaid infrastructure.
 14. The method of claim 1 wherein said glasscontains up to 2 weight percent of a color-imparting metal oxide. 15.The method of claim 1 wherein said glass is a mixture of oxides whichafter solidification provides an essentially amorphous mass which istransparent or provides an essentially crystalline mass which isopalescent.
 16. The method of claim 1 wherein said glass has an index ofrefraction different from an index of refraction of said infrastructure.17. The method of claim 1 wherein to said slip comprising said aqueoussuspension of metal oxide particles of pH control agent has been added.18. The method of claim 1 wherein said molding mass comprises siliconoxide or aluminum oxide together with a binding agent.
 19. The method ofclaim 18 wherein said binding agent is sodium silicate, ethyl silicate,ammonium sulfate, aluminum phosphate or sodium phosphate.
 20. The methodof claim 1 which includes adjusting pH of said slip to about 7 to 8 byaddition thereto of a pH control agent.
 21. The method of claim 20wherein said pH control agent is citric acid.
 22. The process of claim 1wherein said model of said tooth coated with said metal oxide basedinfrastructure is baked initially at a temperature which is raisedslowly at a rate of 1° C./minute up to a temperature of about 350° C.and thereafter raising the temperature to at least equal to about 1050°C. for a time sufficient to effect said fritting of said metal oxidebased infrastructure.
 23. The method of claim 22 wherein during saidfritting said infrastructure undergoes a withdrawal from said model ofsaid tooth of less than 0.4% so as to correspond to a percent expansionof said model of said tooth on solidification of said molding mass. 24.The method of claim 1 wherein an outer surface of said prosthesis iscoated with an enamel compatible with said glass employed to impregnatesaid infrastructure, said enamel having a coefficient of expansion closeto said glass.
 25. The method of claim 24 wherein said enamel has acoefficient of expansion lower than said glass.
 26. The method of claim24 wherein said enamel is an alkali borosilicate containing alumina. 27.The method of claim 1 wherein said metal oxide particles are aluminaparticles.
 28. The method of claim 27 wherein said metal oxide particlesalso include magnesium oxide, lanthanum oxide or rare earth metal oxide.29. The method of claim 27 wherein said metal oxide particles alsoinclude zirconium oxide particles.